Lots of explanation regarding sensor design, capacitance, sensors design, and controllers are presented in the design guide. These are some excerpt from the design guide:

The process for designing products that use touch controls is a complex process with many decisions to be made, such as what materials will be used in their construction and how the mechanical and electrical requirements will be met. Key to this process is the design of the actual sensors (specifically keys, sliders, wheels and touchscreens) that form the interface with the user. Sensor design is often considered a “black art”; the distributed nature of the electric fields between the sensor and its electrical environment can make simple “lumped element” approximations of sensor behavior misleading at best.